So I assume it’s covered in the side stories, to which I have only read a couple of. Apparently it covered Yohans P.O.V. So here’s someone who actually went over it: https://www.mangago.me/home/mangatopic/22050632/
According to this person, Yohan had an incident that involved his father ending up whacking his head with a metal trophy. This causes damages to his brain. I assume it’s TBI (traumatic brain injury) since most of his symptoms and behaviors align so well with it. From that incident, Yohan has had immense changes in attitude and behaviors, He’s more implusive and has heavy mood swings. Which explains his actions towards Jun through the entire novel.
Apparently, he viewed Jun as a ‘lost’ and ‘naive’ lamb and that he thinks he has to save Jun. When Jun pulled that stunt infront of Yohans father, Jun crossed a line. Yohan and the rest of his siblings hold a lot of respect for their father, so I see why that upset him.
So in summary, the main reasons are; Jun crossed a line and heavily embarrassed Yohan infront of the person Yohan respected most. Yohan wants to break Jun down so that he can be Jun’s savior in Jun’s eyes.

https://www.mangago.me/home/mangatopic/21841102/
Spoilers for anyone interested and links for the novel at the end!!